1. Field of the Invention
The present invention relates to high sensitivity spectroscopy.
2. Brief Description of the Related Art
Sensitivity of spectroscopic detection is a crucial issue for applications where trace gas chemicals are to be measured. Atmospheric chemistry, environmental monitoring, biomedical diagnostics and molecular astrophysics are just a few examples of areas of research demanding for ultra-sensitive and rugged setups, capable of field measurements. See, for example, R. M. Mihalcea, M. E. Webber, D. S. Baer, R. K. Hanson, G. S. Feller, W. B. Chapman, “Diode-laser absorption measurements of CO2, H2O, N2O, and NH3 near 2.0 μm,” Appl. Phys. B 67, 283-288 (1998); E. C. Richard, K. K. Kelly, R. H. Winkler, R. Wilson, T. L. Thompson, R. J. Mclaughlin, A. L. Schmeltekopf, A. F. Tuck, “A fast-response near-infrared tunable diode laser absorption spectrometer for in situ measurements of CH4 in the upper troposphere and lower stratosphere,” Appl. Phys. B 75, 183-194 (2002); G. Gagliardi, R. Restieri, G. De Biasio, P. De Natale, F. Cotrufo, and L. Gianfrani, “Quantitative diode laser absorption spectroscopy near 2 μm with high precision measurements of CO2 concentration,” Rev. Sci. Instrum. 72, 4228-4233 (2001); H. Dahnke, D. Kleine, W. Urban, P. Hering, M. Mürtz, “Isotopic ratio measurement of methane in ambient air using mid-infrared cavity leak-out spectroscopy,” Appl. Phys. B 72, 121-125 (2001); L. Menzel, A. A. Kosterev, R. F. Curl, F. K. Tittel, C. Gmachl, F. Capasso, D. L. Sivco, J. N. Baillargeon, A. L. Hutchinson, A. Y. Cho, W. Urban, “Spectroscopic detection of biological NO with a quantum cascade laser,” Appl. Phys. B 72, 1-5 (2001); H. Dahnke, D. Kleine, P. Hering, M. Mürtz, “Real-time monitoring of ethane in human breath using mid-infrared cavity leak-out spectroscopy,” Appl. Phys. B 72, 971-975 (2001); and G. J. German, and D. J. Rokestraw, “Multiplex spectroscopy: determining the transition moments and absolute concentrations of molecular species,” Science 264, 1750-1753 (1994); C. R. Webster, “measuring methane and its isotopes 12CH4, 13CH4 and CH3D on the surface of Mars with in situ laser spectroscopy,” Appl. Opt. 44, 1226-1234 (2005).
In general, for most of spectroscopic methods, the Beer-Lambert law that underpins much of molecular absorption spectroscopy leaves two clear paths to improve the detection sensitivity: increasing the radiation/sample interaction pathlength and reducing the noise that affects the absorption signal.
With regard to the first, there are a number of well-established methods for absorption length enhancement, the most effective of which exploit the several-kilometers paths attainable in high finesse optical cavities. Techniques such as cavity ring-down spectroscopy (CRDS), cavity-enhanced absorption spectroscopy (CEAS) and integrated cavity output spectroscopy (ICOS), demonstrated minimum detectable absorption coefficients ranging from 10−8 cm−1 Hz−1/2 to 10−11 cm−1 Hz−1/2. See, D. Romanini, A. A. Kachanov, N. Sadeghi, and F. Stockel, “CW cavity ring down spectroscopy,” Chem. Phys. Lett. 264, 316-322 (1997) and K. Nakagawa, T. Katsuda, A. S. Shelkovnikov, M. de Labachelerie, and M. Ohtsu, “Highly sensitive detection of molecular absorption using a high finesse optical cavity,” Opt. Commun. 107, 369-372 (1994).
As for noise suppression, different strategies are usually adopted for the various optical and electronic noise components. For example, high speed ensemble averaging can reduce white noise of a detector and its associated electronics while antireflection coatings can suppress the optical noise that often limits the sensitivity of laser-based spectrometers.
Since the 1980's heterodyne detection schemes have been employed to suppress the technical 1/f (flicker) noise associated with all laser sources and most photodetectors. See, J. M Supplee, E. A. Whittaker, W. Lenth, Appl. Opt. 33, 6294 (1994); D. E. Cooper, J. P. Watjen, Opt. Lett. 11, 606 (1986); and G. C. Bjorklund, Opt. Lett. 5, 15 (1980). The general principle underlying these methods consists of modulating the laser current to produce a pair of sidebands on the laser carrier and, after interaction with the sample, detecting and demodulating their beat frequency by means of phase-sensitive electronics. In this way, a sample's absorption depth and linewidth are encoded at the modulation frequency, which can be set in the radiofrequency (RF) domain, where the flicker noise is reduced by a few orders of magnitude. Many variations of this basic heterodyne detection scheme have been successfully applied, providing in some cases sensitivity enhancements up to two orders of magnitude with respect to traditional laser spectroscopy. See, P. Werle, F. Slemr, M. Gehrtz, and C. Bräuchle: Appl. Phys. B 49, 99 (1989); J. A. Silver: Appl. Opt. 31, 707 (1992); and P. Werle: Spectrochim. Acta Part A 54, 197 (1998).
Additionally, in U.S. Pat. No. 6,795,190, a method and apparatus was disclosed in which a continuous wave light beam was introduced into a cavity using off-axis cavity alignment to systematically eliminate optical resonant noise commonly associated with optical cavities while preserving the absorption signal amplifying properties of such cavities.
The realization of a technique that combines the signal enhancement of cavity-based techniques with the noise suppression of a heterodyne detection scheme has proven non-trivial. It was addressed only in 1998 by Ye et al., with a technique called noise-immune cavity-enhanced optical-heterodyne molecular spectroscopy (NICE-OHMS). See, J. Ye, Long-Sheng Ma, and J. L. Hall, “Ultrasensitive detections in atomic and molecular physics: demonstration in molecular overtone spectroscopy” J. Opt. Soc. Am B 15, 6-15 (1998). In NICE-OHMS, the radiation injected in the optical resonator is modulated at a frequency that is a multiple of the cavity mode spacing (FSR), thus creating a couple of sidebands available at the cavity output. Then, the transmitted signal is heterodyned to retrieve information on the intracavity absorption. In this way, an unprecedented shot-noise limited detection sensitivity, as low as 10−14 cm−1 Hz−1/2 was demonstrated in the near infrared. However, in order to be transmitted by the cavity, both carrier and modulation sidebands must be constantly in resonance with three cavity modes. To achieve this condition, the cavity mode spacing or free spectral range (FSR) and the laser frequency need to be stabilized independently by means of tight, fast, and low-noise frequency-locking schemes. As a consequence, the setup is technically extremely demanding and particularly sensitive to mechanical shocks and vibrations. This makes the extraordinary high sensitivity reachable by NICE-OHMS only attainable in laboratory environments.